1. Field of the Invention
The present invention relates to a light-receiving and amplifying device for use in a pickup for a mini-disc player, a magneto-optic disc player, a multi-disc player, or the like.
2. Description of the Prior Art
There has been conventionally provided a photodiode for a light-receiving and amplifying device as shown in FIGS. 1 and 2, where a rectangular N-type epitaxial layer 3 is formed as isolated from its periphery by a P-type isolation diffusion layer 2, and four rectangular P-type diffusion layers 4a, 4b, 4c, and 4d are arranged in a lengthwise direction on the N-type epitaxial layer 3. Between a bottom surface of the rectangular N-type epitaxial layer 3 and the P-type isolation diffusion layer 2 is formed an N.sup.+ buried layer 5. The N-type epitaxial layer 3 and the P-type diffusion layers 4a, 4b, 4c, and 4d are forming four photodiodes PD.sub.A, PD.sub.B, PD.sub.C, and PD.sub.D. The photodiodes PD.sub.A through PD.sub.D structurally have a common cathode. Between the P-type isolation diffusion layer 2 and the N-type epitaxial layer 3 there is a parasitic photodiode PD.sub.E, and the parasitic photodiode PD.sub.E structurally has its cathode common to the cathodes of the photodiodes PD.sub.A through PD.sub.D. A connector compensation diffusion layer 6 for separating a peripheral region 3a of the N-type epitaxial layer 3 from an inner region 3b which includes the inner P-type diffusion layers 4a through 4d is further formed to lead the common cathode of the photodiodes PD.sub.A through PD.sub.D and the parasitic photodiode PD.sub.E to the outside thereof through the connector compensation diffusion layer 6.
FIG. 3 shows an equivalent circuit diagram of a light-receiving and amplifying device employing the photodiodes PD.sub.A through PD.sub.D. The cathodes of the photodiodes PD.sub.A through PD.sub.D and the cathode of the parasitic photodiode PD.sub.E are connected to a power source V.sub.CC. The anodes of the photodiodes PD.sub.A through PD.sub.D are connected to amplifiers AMP.sub.A through AMP.sub.D respectively, while the anode of the parasitic photodiode PD.sub.E is connected to the ground GND. When the photodiodes PD.sub.A through PD.sub.D receive incident light, photocurrents IPD.sub.A, IPD.sub.B, IPD.sub.C, and IPD.sub.D flow through the photodiodes. Then the amplifiers AMP.sub.A through AMP.sub.D convert the photocurrents IPD.sub.A through IPD.sub.D into voltages and amplify the voltages to form voltage outputs V.sub.A, V.sub.B, V.sub.C, and V.sub.D. When the parasitic photodiode PD.sub.E receives incident light, a photocurrent IPD.sub.E also flows through the parasitic photodiode.
A pickup employing the light-receiving and amplifying device having the above-mentioned construction reproduces a signal recorded on a disc or the like through signal processing such as calculation based on the voltage outputs V.sub.A through V.sub.D.
In the pickup of a mini-disc player, a magnetooptic disc player, a multi-disc player, or the like, the quantity of light incident on the photodiodes changes depending on whether the player is in the reproduction stage or in the recording stage. In practice, the quantity of incident light in the recording stage is greater than in the reproduction stage. Furthermore, since the reflectance differs depending on the type of the disc, the quantity of incident light varies. The performance of the pickup depends on how much the signal-to-noise ratio can be increased when the quantity of incident light is at minimum and the signal component is small in condition where the quantity of incident light varies. The noise level for determining the signal-to-noise ratio varies according to the feedback resistor of the amplifier circuit. Assuming that the input current of the amplifier circuit is iS and the equivalent input current noise is iN, the equivalent input noise current can be obtained by the mean square of a thermal noise iNR due to the feedback resistor of the amplifier circuit and the other noise iNS such as shot noise. Assuming now that the feedback resistor of the amplifier circuit is Rf, the thermal noise iNR due to the feedback resistor Rf can be expressed as: EQU iNR=4KTB/Rf
where K is the Boltzmann's constant of 1.38.times.10.sup.-23, J/.degree.K., T is the absolute temperature, and B is a bandwidth. Therefore, the signal-to-noise ratio S/N of the amplifier circuit can be expressed as: ##EQU1## which results in an increase of the signal-to-noise ratio S/N as the resistance of the feedback resistor Rf increases. However, when a gain of each of the amplifiers AMP.sub.A through AMP.sub.D is increased according to the small quantity of incident light in the reproduction stage, the outputs V.sub.A through V.sub.D of the amplifiers AMP.sub.A through AMP.sub.D are possibly saturated when the quantity of incident light is increased. In contrast, when the gain of each of the amplifiers AMP.sub.A through AMP.sub.D is reduced in accordance with the great quantity of incident light in the recording stage, the signal-to-noise ratios S/N of the outputs V.sub.A through V.sub.D of the amplifiers AMP.sub.A through AMP.sub.D are problematically deteriorated when the quantity is of incident light is reduced.